It is possible to formulate and fabricate semiconductor materials by doping crystal lattice materials such that an amount of an element belonging to one column on the periodic table of the elements, (e.g., an element having one number of conduction or outer shell electrons), is replaced with an element from a different column or group on the periodic table, (e.g., an element having a different number of conduction or outer shell electrons, usually one column or group removed such that it has one more or one fewer outer shell electrons). It is also possible to use various alloys to form the semiconductor material, (i.e., substitutions on lattice sites by elements from the same column (or group) in the periodic table), to obtain particular semiconductor characteristics as needed or desired. For example, particular band gaps, crystal lattice constants, mobility, and the like may be obtained.
U.S. Pat. No. 6,815,736 to Mascarenhas (Mascarenhas), builds on the teachings discussed above, describing a system for isoelectronic co-doping of semiconductor compounds and alloys with deep acceptors and deep donors to decrease bandgap, to increase concentration of the dopant constituents in the resulting alloys, and to increase carrier mobility and lifetimes. Group III-V compounds and alloys, such as GaAs and GaP, are isoelectronically co-doped with, N and Bi, to customize solar cells, thermophotovoltaic cells, light emitting diodes, photodetectors, and lasers on GaP, InP, GaAs, Ge, and Si substrates. The GaAs:N:Bi compound discussed in Mascarenhas has great promise; however, difficulties have been recognized in practical implementations. For example, Nitrogen acting as a deep acceptor, introduces impurity bands in the vicinity of the conduction band of GaAs, which degrades the electron transport in this band. Thus, the mobility of electrons may be lower than optimal. At low N and Bi doping levels (typically <1%), the statistical probability for the occurrence of N—Bi pairs is very low. Therefore, the benefits attributed to charge and size balancing of the N and Bi isoelectronic impurities using co-doping that were anticipated in Mascarenhas are difficult to attain in practice.
Similarly to Mascarenhas, but in the context of transistors, U.S. Pat. No. 6,936,871 to Hase (Hase), discusses a heterojunction bipolar transistor (HBT) employing a Group III-V compound semiconductor having Bi added thereto used for a base layer of a GaAs-based or InP-based HBT. For example, Hase describes a GaAs-based HBT formed by various layers, including a GaAsBi:N base. However, Hase does not appear to recognize that the proposed emitter base junction formed at a boundary between the GaAs emitter and GaAsBi:N base may have undesirable electrical transport characteristics. To wit, co-doping of GaAs with Bi and N will have the beneficial effect of a Bi doping induced valence band offset (and thus a desirable barrier for blocking back injection of holes from the base to emitter), and the counterbalancing of the concomitant Bi doping induced lattice mismatch, by the N co-doping; however, Hase does not recognize that the impurity bands in the vicinity of the conduction band, that are inherently associated with N doping, may not be mitigated by the Bi co-doping for the reasons discussed above. Hence, electron transport in the HBT structure of Hase including GaAsBi:N, will not be optimal.